This invention relates to strontium ferrite permanent magnets and, more particularly, to a more economical and energy-saving method of making high quality boron-containing strontium ferrite permanent magnets.
One technique for making a strontium ferrite permanent magnet is disclosed in U.S. Pat. No. 3,855,374 Brailowsky et al., entitled "Method of Making Magnetically Anisotropic Permanent Magnets", issued Dec. 17, 1974. Normally, high purity ferric oxide of small grain size is used as a starting material to obtain hard ferrite magnets with the highest energy product and coercive force. Brailowsky et al. disclose that high quality strontium ferrite permanent magnets can be made with a low cost natrual magnetite having a relatively large grain size, if a small proportion of boric acid or boric oxide is included in the reactant mixture. Brailowsky et al. disclose milling the low cost iron oxide, strontium carbonate and a minor proportion of boric acid as a water slurry to form a homogeneous mixture. The mixture is dried, screened and tumbled to form a free-flowing powder. The powder is then calcined at about 2100.degree. F. to form a ferrite. The ferrite is comminuted in water to a particle size that is ceramically workable and pressed into a body of desired shape, preferably while under the influence of a directional magnetic field. The pressed body is then sintered at a temperature in the range of 2000.degree. - 2150.degree. F. to form a permanent magnet having the desired high magnetic properties.
During milling in the Brailowsky et al. process the reactants are dispersed in water, typically about 50% by weight water. This water must be removed before calcining. However, it cannot be removed by mechanical separation because the boric acid and/or boric oxide are dissolved in it. Instead, the water is removed by evaporation. However, during slow evaporation the boron compound can segregate nonuniformly. This may require further mixing of the dry material before calcining. To avoid this extra step, the slurry is normally flash dried. Flash drying results in uniform boron compound distribution in the reactant mixture. Removing the water by evaporation, alone, consumes a large amount of energy. For example, starting at room temperature, more than twice the amount of energy is required to evaporate the water in the typical slurry than to calcine the reactants. Moreover, the heat losses in flash drying makes the evaporation energy requirement even greater.
I have recognized that removing the water from the slurry by mechanical means, requires considerably less energy than flash drying. By mechanically removing the water, I mean to separate the water from the reactants without a physical change, a change in state, or a chemical change of either the water or the reactants. If so, considerable energy savings could be achieved. Only a comparatively small amount of heat would be needed to remove remaining moisture after the mechanical separation of water, and it can be applied more efficiently than in flash drying. Thus, natural gas could be conserved.
If an insoluble source of boron oxide is used as a substitute for boric acid and/or boric oxide in the Brailowsky et al process, most of the water in the slurry can be mechanically removed, and flash drying avoided. Ferro-boron is one such source. Ferro-boron is particularly satisfactory since it oxidizes below calcining temperatures and is completely compatible with the other ferrite reactants. While high in cost, such a small proportion is used that a cost savings, due to energy savings, also can be realized. Since the ferro-boron oxidizes at a temperature below normal calcining temperatures, the residue can be calcined and further treated in the normal and accepted manner. No other changes in equipment or process steps are required.